Virus-like particles (VLPs) are sophisticated multiprotein complexes that emulate the structural organization and conformation of authentic viruses but lack the ability to infect or replicate within host cells. VLPs inherit many immunogenic characteristics from their native viral counterpart. While replication-competent viral vectors have been utilized as vaccine carriers in both clinical and preclinical contexts, their application is often limited to those vectors capable of infecting vertebrate cells, and often raises safety concerns. VLPs are considered promising alternatives due to their favorable safety profile, the diverse array of viral strains available for exploitation and the enhanced flexibility for engineering. Qβ VLP is derived from Escherichia coli phage Qβ and its structure and immunogenicity have been extensively characterized. The versatility of the platform enables a broad scope of antigens, including peptides, proteins and carbohydrates. It also allows rational design and optimization of the antigen structure loaded on the carrier. In the first chapter of this thesis, recent advances of viral vectored vaccines in cancer immunotherapy are reviewed. In the second and third chapters of this thesis mutant Qβ (mQβ) VLPs are used as vaccine carriers against some emerging human pathogens: SARS-CoV-2 and antibiotic resistant Staphylococcus Aureus. To develop a broad and long lasting COVID vaccine, SARS-CoV-2 Spike protein receptor binding domain (RBD) based epitopes are investigated as conjugates with mQβ. The epitope design is critical to eliciting potent antibody responses with the full length RBD being superior to peptide and glycopeptide antigens. The full length RBD conjugated with mQβ activates both humoral and cellular immune systems in vivo, inducing broad spectrum, persistent and comprehensive immune responses effective against multiple variants of concerns including Delta and Omicron, rendering it a promising vaccine candidate. For S. Aureus vaccine development, oligosaccharides derived from poly-β-(1−6)-N-acetylglucosamine (PNAG), a polysaccharide expressed on the surface of numerous pathogens, are used for antigen design. A significant challenge in the development of a PNAG-based vaccine lies in the incomplete understanding of the influence of the number and positional arrangement of free amines versus N-acetylation on the antigenicity of PNAG. A divergent strategy is developed to synthesize a comprehensive library of 32 PNAG pentasaccharides. This library enables the identification of PNAG sequences with specific patterns of free amines as epitopes for vaccines against S. aureus. Active vaccination with the conjugate of discovered PNAG epitopes with mQβ as well as passive vaccination with diluted rabbit antisera provides mice with near complete protection against infections by S. aureus including methicillin-resistant S. aureus (MRSA). In conclusion, mQβ is a promising platform for next generation vaccine design due to its flexibility to present diverse antigens and its capacity to augment both humoral and cellular immunity.
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